1. Field of the Invention
The present invention relates to a fill-up control valve device that is provided in a ventilation passage that communicably connects an interior space and an exterior space of a fuel tank.
2. Description of Related Art
Fill-up control valve devices, as an art related to the present invention, include a float valve in a casing mounted to a fuel tank. The float valve floats on liquid fuel. When a fuel surface in the fuel tank rises and reaches a first predetermined level or a fill-up level, a surface of fuel introduced from a lower vent hole into the casing is pressed up to a second predetermined level higher than the first predetermined level, so that the float valve is seated on a valve seat formed in an upper portion of the casing to close a ventilation passage that communicates between an interior and an exterior of the fuel tank.
Thus, when the fuel surface in the fuel tank rises and reaches the fill-up level through refueling from a refueling port, a pressure in the fuel tank increases. Accordingly, a fuel surface in a refueling pipe rises close to the refueling port. Additional refueling over the fill-up level is thereby prevented.
One of these known fill-up control valve devices has a minute vent hole for the purpose of reducing variations in pressure in the fuel tank after the fuel tad is filled up. The vent hole is formed on the casing at a location above the foregoing second predetermined level to communicate an interior space of the fuel tank with an interior space of the casing (see, for example, Japanese Patent Application Publication No. 2004-257264 (JP 2004-257264 A) described below).
However, in the fill-up control valve device according to the related art, a ventilation passage that communicates between an interior and an exterior of the fuel tank is blocked when a fuel surface in the fuel tank reaches the fill-up level positioned at a lower end of the casing. Consequently, the pressure in the fuel tank is higher than the pressure in the casing, and the increase in pressure in the fuel tank causes the fuel surface in the casing to be pressed up so that a float valve closes the ventilation passage. However, because the vent hole is provided on the casing above the foregoing second predetermined level or the level of the pressed-up fuel surface, an air pressure in the casing and an air pressure in the fuel tank are balanced gradually. Liquid fuel in the casing is thereby drained from a lower vent hole. The float valve opens the ventilation passage relatively shortly after closing the ventilation passage. This causes the pressure in the fuel tank to decrease. Then, the fuel surface in the refueling pipe falls from the level close to the refueling port. This undesirably allows additional refueling although fuel tank is filled to the fill-up level, which is a problem. Therefore, desirably the float valve is kept in a closed state for an extended period of time in order to reliably prevent additional refueling after filling up the fuel tank.